The invention relates to a control system for a continuously variable transmission.
Toroidal type continuously variable transmissions have recently been practiced. Such a toroidal type continuously variable transmission includes a roller interposed between input and output toroidal disks. Gear ratio of the CVD is varied by inclining the roller relative to the input and output toroidal disks to continuously vary the rate of torque transmission between the input and output toroidal disks. One of this type of toroidal type CVDs, such as described in, for example, Japanese Unexamined Patent Publications Nos. 3-223555 and 6-101754, is characterized in what is called a geared neutral starting system. In the toroidal type continuously variable transmission equipped with this system, the toroidal continuously variable transmission mechanism is mounted on a transmission input shaft connected to the engine, and a planetary gear set is mounted on a secondary shaft in parallel to the input shaft. The planetary gear set is comprised of three rotary elements, namely a sun gear, an internal gear and a pinion carrier supporting a pinion gear meshed with the sun gear and the internal gear. One of these rotary elements, i.e. the internal gear is used as an transmission output gear. Rotation of engine is imparted to the planetary gear set partly directly through the pinion carrier and partly through the sun gear via the toroidal continuously variable transmission mechanism.
The ratio of rotation between the pinion carrier and the sun gear is varied by controlling the gear ratio of the toroidal type continuously variable transmission mechanism so as to hold the transmission output element, i.e. the sun gear, remain stand still, providing a neutral condition. By increasingly or decreasingly varying the gear ratio of the continuously variable transmission mechanism causes the internal gear as the transmission output element to rotate in a forward direction or in a reverse direction. This type of toroidal continuously variable transmission permits the vehicle to start forward or back without use of a clutch and/or torque converter, which yields improvement of responsibility and power transmission efficiency of the toroidal continuously variable transmission.
The toroidal continuously variable transmission mechanism is controlled to vary its gear ratio so as to gain a target overall transmission gear ratio or to provide a target engine speed which is obtained from a control map which defines gear ratio change patterns according to vehicle speeds and throttle positions. As is known in the art, the gear ratio of the toroidal continuously variable transmission mechanism is varied by controlling angles of the roller with respect to the toroidal surface of the input and output disks. For this control, the toroidal continuously variable transmission mechanism is provided with two hydraulic piston chambers extending in a direction in which a trunnion moves with respect to the input and output disks to incline the roller, these hydraulic piston chambers being supplied with hydraulic line pressure and relief pressure, respectively. The trunnion is moved according to the difference in pressure between these hydraulic piston chambers. Double-slider shift valves are disposed in hydraulic lines leading to the hydraulic piston chambers from a line pressure regulation valve and a relief valve regulation valve, respectively. The pressure difference between the two hydraulic piston chambers is varied by controlling these double-slider valves.
The double-slider valve, which is generally comprised of a valve body, a valve sleeve movable in the valve body and a valve spool movable in the valve sleeve, shifts the sleeve to control and regulate the line pressure or the relief pressure so as to vary the pressure difference between the two hydraulic piston chambers. Accordingly, it is desirable that the shift distance of valve sleeve and the pressure difference unconditionally accord with each other for precise gear ratio control. While if there is an unconditional relationship between them, controlling the shift distance of the valve sleeve of a double-slider valves causes an overall range of gear ratios of the toroidal continuously variable transmission mechanism, whereas, it is difficult to control the shift distance of the valve sleeve due to frictional distance which is usually different according to directions in which the valve sleeve shifts, as a result of which the same shift distance does not always provide the same pressure difference. That is, the movement of the valve sleeve is subject to a hysteresis too significant to make desirably precise control of the pressure difference. In particular, in the event where the desired pressure difference is not developed at a geared neutral position which is a switching point for a directional reversal of output rotation of the toroidal continuously variable transmission mechanism, the toroidal continuously variable transmission mechanism causes reverse rotation even while the vehicle is moving forward at a low speed or forward rotation even while moving back at a low speed.